The present invention relates to a desalination apparatus that obtains fresh water from raw water, e.g. sea water, ground water (brine) containing salt, or industrial waste water, by a distillation method utilizing solar energy, and also relates to a method of operating the desalination apparatus.
Recently, great interest has been focused on the technique of desalinating sea water by utilizing solar energy. FIG. 17 is a schematic sectional view of a conventional desalination apparatus utilizing solar energy, which is known as xe2x80x9cbasin type solar heat distillerxe2x80x9d. The desalination apparatus in FIG. 17 has a basin 102 for holding raw water 101, e.g. sea water, and an air shield 103 that covers the basin 102 and is permeable to solar energy 104. The outer surface of the air shield 103 is used as a radiating part 106. In the desalination apparatus in FIG. 17, the raw water 101 in the basin 102 is heated with solar energy 104 to generate water vapor 105. The water vapor 105 is cooled and condensed on the inner surface of the air shield 103, and thus distilled water 107 is obtained.
Solar energy (sunlight) is a massive energy source in terms of quantity. However, the energy density of sunlight is extremely low, i.e. 1 kw/m2 at most, and it varies considerably according to hours and seasons. For these reasons, solar energy cannot be applied to industrial techniques that require stable high-density energy. Desalination apparatuses utilizing solar energy, which have been proposed or developed so far, including the above-described basin type solar heat distiller, suffer low efficiency and are inferior in durability. Therefore, the conventional desalination apparatuses are not suitable for practical application.
The present invention was made in view of the above-described problems, and it is an object of the present invention to provide a desalination apparatus utilizing solar energy that is capable of obtaining fresh water from raw water by satisfactorily grasping the characteristics of solar energy and effectively utilizing it, and also provide a method of operating the desalination apparatus. A particular object of the present invention is to provide a desalination apparatus capable of obtaining a large amount of distilled water at low temperature by utilizing a depressurized condition. Another object of the present invention is to increase the useful life of a solar heat collector by using a circulating heating medium. A further object of the present invention is to provide a multiple-effect desalination apparatus having a combination of evaporation cans and condensers, which can utilize with high efficiency thermal energy obtained in a solar heat collector. A still further object of the present invention is to provide a desalination apparatus that consumes very little power so that it can be driven by electric power from a solar battery. A still further object of the present invention is to make the structure of a multiple-effect desalination apparatus compact in size and simplified. In addition, an object of the present invention is to provide a simplified method of operating a multiple-effect desalination apparatus with high efficiency. Other objects and advantages of the present invention will be made apparent from the following description of embodiments, taken in connection with the drawings, and in the appended claims.
A desalination apparatus according to the present invention has a solar heat collector for heating a heating medium with solar energy, a heat exchanger cooperating with an evaporation can so as to subject the heating medium and raw water in the evaporation can to heat exchange and generate water vapor in the evaporation can, a condenser cooperating with a raw water tank so as to receive the water vapor from the evaporation can, cool the water vapor by subjecting the water vapor and raw water in the raw water tank to heat exchange and obtain distilled water, a distilled water tank for storing distilled water, vacuum means for evacuating the evaporation can and depressurizing the inside of the evaporation can so as to promote generation of water vapor in the evaporation can, and raw water supply means for supplying raw water to the evaporation can. The vacuum means depressurizes the inside of the evaporation can, a condensation space communicated with the inside of the evaporation can, a distilled water storing space, etc.
The desalination apparatus according to the present invention preferably has a plurality of evaporation cans arranged in series, i.e. in a multiple-effect relation to each other, and a condenser cooperating with the evaporation cans. In this case, the heat exchanger is arranged to cooperate with a first evaporation can so as to generate water vapor in the first evaporation can, and the condenser cooperating with the raw water tank is arranged to receive water vapor from a final evaporation can. The condenser cooperating with the evaporation cans receives water vapor from the upstream-side evaporation can, cool the water vapor with raw water in the downstream-side evaporation can and thereby produce distilled water, and also heat the raw water in the downstream-side evaporation can and generate water vapor.
More specifically, in a case where two evaporation cans arranged in series are used, the heat exchanger cooperates with the upstream-side evaporation can to generate water vapor in the upstream-side evaporation can. The condenser cooperating with the raw water tank is arranged to receive water vapor from the downstream-side evaporation can, cool the water vapor with raw water in the raw water tank and thereby produce distilled water. The condenser cooperating with the evaporation cans is disposed so as to receive water vapor from the upstream-side evaporation can, cool the water vapor with raw water in the downstream-side evaporation can and thereby produce distilled water, and also heat the raw water in the downstream-side evaporation can and generate water vapor.
In a case where three evaporation cans, i.e. a first, second and third evaporation cans, are used in series, two pairs of upstream- and downstream-side evaporation cans are formed, and two condensers cooperating with the evaporation cans are used. More specifically, the heat exchanger cooperates with the first evaporation can, and the condenser cooperating with the raw water tank is arranged to receive water vapor from the third evaporation can, cool the water vapor with raw water in the raw water tank and thereby produce distilled water. One of the condensers cooperating with the evaporation cans is disposed so as to receive water vapor from the first evaporation can, cool the water vapor with raw water in the second evaporation can and thereby produce distilled water, and also heat the raw water in the second evaporation can and generate water vapor. The other of the condensers cooperating with the evaporation cans is disposed so as to receive water vapor from the second evaporation can, cool the water vapor with raw water in the third evaporation can and thereby produce distilled water, and also heat the raw water in the third evaporation can and generate water vapor. In a case where N evaporation cans (N is an integer of 4 or higher) are used in series, Nxe2x88x921 pairs of upstream- and downstream-side evaporation cans are formed, and Nxe2x88x921 condensers are used to cooperate with the evaporation cans.
The desalination apparatus according to the present invention preferably has the following arrangements. (a) The heating medium is circulated by the action of thermosiphon in which the heating medium is heated in the solar heat collector to form vapor, which is then cooled to become liquid in the first evaporation can. (b) The heating medium is water. (c) The desalination apparatus has an air shield that covers the raw water tank, the air shield having a structure in which the inner surface of the air shield cools water vapor to form distilled water and collects it, the outer surface of the air shield being adapted to be a radiating part. (d) The energy collecting part of the solar heat collector is placed in a part of the upper outer surface of the air shield on which sunlight impinges. (e) At least a heating part of the evaporation can, which is constructed by disposing the heat exchanger or the heat transfer tube of the condenser, is placed in the air shield. (f) The solar heat collector is placed to cover the upper part of the whole desalination apparatus so as to intercept sunlight directed toward the other part of the desalination apparatus. (g) The heat exchanger has a heat transfer tube extending approximately horizontally in the evaporation can, the heating medium being passed through the heat transfer tube, and a heating part and evaporation part for the raw water are formed between the inner surface of the evaporation can and the outer surface of the heat transfer tube. Similarly, the condenser in the evaporation can has a heat transfer tube extending approximately horizontally in the evaporation cans, the water vapor being passed through the heat transfer tube, and a heating part and evaporation part for the raw water are formed between the inner surface of the evaporation can and the outer surface of the heat transfer tube. (h) The heat transfer tube is tilted slightly with respect to the horizontal direction so that the heating medium or vapor inlet side is above the heating medium or vapor outlet side. (i) The condenser cooperating with the raw water tank has a heat transfer tube extending approximately parallel and adjacent to the bottom surface of a the raw water tank. (j) The raw water tank is capable of containing raw water to a depth at which a thermal stratification is formed. (k) The raw water tank has an air diffuser tube that supplies air bubbles into the raw water. Preferably, the air diffuser tube is placed so as to supply air bubbles into the raw water near the surface of the raw water. (1) The desalination apparatus has solarlight power generation equipment, so that the desalination apparatus is driven by electric power supplied from the solarlight power generation equipment.
The desalination apparatus according to the present invention can selectively have the following arrangements. (m) A gap is provided between the energy collecting part of the solar heat collector, which is disposed along the upper outer surface of the air shield, and the upper outer surface of the air shield, and draft is induced in the gap by the stack effect. (n) The heat transfer tube is disposed to extend approximately parallel to the bottom surface of each evaporation can at a position not higher than xc2xd of the height of the evaporation can. (o) The amount of water held in the raw water tank is set so that the rise in temperature caused by cooling water vapor is not more than 10xc2x0C. per day.
In a desalination apparatus operating method according to the present invention, the desalination apparatus has a solar heat collector for heating a heating medium with solar energy, a plurality of evaporation cans, a raw water tank, a distilled water tank, a condenser disposed in the raw water tank, vacuum means for evacuating the evaporation cans and depressurizing the insides of the evaporation cans, and raw water supply means for supplying raw water to the evaporation cans.
The desalination apparatus operating method according to the present invention has the step of supplying a predetermined amount of raw water into the evaporation cans by operating the raw water supply means, the step of evacuating the evaporation cans to produce a predetermined degree of vacuum in the evaporation cans by operating the vacuum means, the step of conveying solar energy to the raw water in a first evaporation can from the solar heat collector through the heating medium to evaporate the raw water in the first evaporation can, the step of successively cooling water vapor generated in an upstream-side evaporation can with raw water in a downstream-side evaporation can to condense the water vapor to distilled water and collecting the distilled water in the distilled water tank and further generating water vapor in the downstream-side evaporation can, the step of cooling water vapor generated in a final evaporation can with raw water in the raw water tank to condense the water vapor to distilled water and collecting the distilled water in the distilled water tank, the step of taking out the distilled water collected in the distilled water tank, and the step of discharging concentrated raw water from the evaporation cans.
The operating method according to the present invention preferably has the following arrangements. (p) The step of supplying a predetermined amount of raw water into the evaporation cans and the step of evacuating the evaporation cans to produce a predetermined degree of vacuum in the evaporation cans by operating the vacuum means are started simultaneously. By doing so, the power consumed by the vacuum means can be reduced. (q) The step of taking out the distilled water collected in the distilled water tank, the step of discharging concentrated raw water from the evaporation cans, the step of supplying a predetermined amount of raw water into the evaporation cans by operating the raw water supply means, and the step of evacuating the evaporation cans to produce a predetermined degree of vacuum in the evaporation cans by operating the vacuum means are carried out and completed in a period of time before sunrise. These are preparatory operations for starting a distilled water producing operation, and it is desirable that these operations be carried out together before sunrise from the viewpoint of efficiency. However, the preparatory operations may be carried out periodically at a fixed time every day or every predetermined number of days according to the circumstances. It is also possible to perform a timer-controlled operation for the purpose of eliminating the need for labor before sunrise.
The operating method according to the present it invention can selectively have the following arrangement. (r) When the amount of distilled water collected in the distilled water tank exceeds a predetermined quantity or it becomes necessary to take out the distilled water during the operation of the desalination apparatus, the communication between the distilled water tank and the condenser is cut off. Then, the distilled water tank is opened to the atmospheric air, and the distilled water is taken out. The inside of the emptied distilled water tank is depressurized by the vacuum pump. Thereafter, the distilled water tank is communicated with the condenser.
According to the present invention, a preparatory process described below is carried out before a distilled water producing operation of the desalination apparatus is started. First, the atmospheric port is opened, and when the pressure in the internal space in the apparatus, which is formed as the inside of the evaporation can, the inside of the condenser and the inside of the distilled water tank are communicated with each other, reaches a level approximately equal to the atmospheric pressure, distilled water produced by the preceding operation in allowed to flow out of the distilled water tank, and raw water left in the evaporation can is discharged. Next, the atmospheric port, the outlet port of the distilled water tank and the raw water discharge port of the evaporation can are closed, and thus the internal space is closed. Then, the internal space is evacuated by operating the vacuum means to produce a predetermined reduced-pressure condition therein. At this time, a predetermined amount of raw water in the raw water tank is supplied into the evaporation can by operating the raw water supply means. By supplying the raw water into the evaporation can while evacuating it, the raw water is allowed to flow into the evaporation can, and the raw water in the evaporation can is degassed by the reduced-pressure condition in the evaporation can. When a predetermined amount of raw water has been supplied into the evaporation can, the raw water supply port is closed. The degree of depressurization of the internal space in the apparatus is determined by taking into consideration the power required for depressurization and the water vapor generation efficiency by the low-temperature heat source. After the pressure in the internal space has been reduced to a predetermined level, the communication between the internal space and the vacuum means is cut off, and the operation of the vacuum means is stopped.
After the completion of the above-described preparatory process, the following distilled water producing operation is carried out. The heating medium is heated with solar energy in the solar heat collector. Preferably, the heating medium is changed into heating medium vapor. The raw water in the evaporation can is heated by the heating medium through the heat exchanger to generate water vapor. In a case where there is only one evaporation can, water vapor in the evaporation can is cooled and condensed by the raw water in the raw water tank through the condenser cooperating with the raw water tank to produce distilled water, which is then collected in the distilled water tank. In a case where N evaporation cans are arranged in a multiple-effect structure, Nxe2x88x921 pairs of upstream- and downstream-side evaporation cans are formed. In each of the Nxe2x88x921 pairs, water vapor in the upstream-side evaporation can is cooled with raw water in the downstream-side evaporation can through the condenser cooperating with the evaporation cans, thereby producing distilled water. At the same time, the raw water in the downstream-side evaporation can is heated to generate water vapor. Water vapor in the final evaporation can is introduced into the condenser disposed in the raw water tank, in which it is cooled to become distilled water, which is then collected in the distilled water tank.